Alexander Hug added that the humanitarian situation there was "relatively catastrophic".

"The local population reported to us that there is no water, no food, no gas, no heating, no electricity, no medication. And all the buildings that our monitors have seen (...) have been affected by the fighting," he said.

Image copyrightReutersImage caption
During the rally in Kharkiv, people held up portraits of local residents killed in the fighting

Image copyrightReutersImage caption
Moves are under way in the east to start pulling back heavy weapons

Media captionPaul Adams reports on the moment Ukrainian prisoners of war were released

Analysis: David Stern, BBC News, Kiev

An amateur video shows a procession of a few hundred people, marching down one of Kharkiv's main streets.

And then the blast. The crowd scatters, terrified. A man writhes in agony on the ground. Another lies lifeless on the snow.

There have been other bomb attacks and explosions in Kharkiv in recent weeks, but this was the deadliest so far.

Moscow-backed rebels have threatened to expand their operations there. Many fear the fighting is now spreading to this strategic city, Ukraine's second largest, which is just a half-hour's drive from the Russian border.

The explosion in Kharkiv happened at 13:20 (11:20 GMT) as people gathered near the city's Palace of Sport for a march in support of national unity, Ukrainian media say.

Officials initially said an explosive device had been thrown from a car but later said it had been buried in the snow.

Eyewitness Alexei Grechnev told the BBC: "We were walking in a column of people, in the front section, when I heard a loud boom and saw some people fall to the ground a few metres away from me, a somewhat unexpected and unreal scene, like in movies."

Image copyrightGetty ImagesImage caption
Thousands rallied in Kiev for the largest of several "dignity marches"

Image copyrightGetty ImagesImage caption
European leaders also attended the march

"Security service detained persons who may have been involved in the preparation and carrying out of crimes of a terrorist nature in Kharkiv, including the explosion," security spokesman Markian Lubkivskyi wrote on his Facebook page.

He later said the four suspects were Ukrainian citizens who had received instruction and weapons in the Russian city of Belgorod, just across the border.

He also posted a picture of a rocket launcher which he said the suspects were planning to use in attacks in the city.

Ukrainian President Petro Poroshenko described the attack as "a bold attempt to expand the territory of terrorism" and promised to bring the perpetrators to justice.

Before Sunday's explosion, Kharkiv, in the north-east of Ukraine, had seen more than a dozen attacks over three months, including an explosion in a bar used by pro-government activists which injured more than 10 people in November.

That attack was blamed on a pro-Russian group calling itself the Kharkiv Partisans.

Sombre mood

Meanwhile, thousands of Ukrainians have been taking part in "dignity marches" in the capital Kiev and other cities, remembering the victims of sniper fire during protests last February.

European leaders including European Council president Donald Tusk, German President Joachim Gauck and the leaders of Lithuania, Poland and Moldova have been attending in Kiev.

The BBC's David Stern in Kiev says the mood was peaceful but sombre.

Thousands marched along a route that followed the main battle zones between anti-government protesters and riot police, a year ago.

More than 100 people died, in what was, until then, the worst violence in Ukraine's history as an independent state.

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